The preferred embodiments concern a method and a device for controlling the circulation speed of an endless belt, in which an endless belt is guided over at least two rollers. The belt is driven by a least one of the rollers with a preset first circulation speed.
In electrophotographic printer or copiers, a print image is electrophotographically generated on a photoconductor, for example an OBC belt (organic photo conductor-photoconductor) in that a charge image is generated on the photoconductor with the aid of a character generator and subsequently developed with toner. The toner image is then transferred onto a belt-shaped intermediate carrier with defined electrical properties. The intermediate carrier can, for example, be a transfer belt.
The toner image located on the intermediate carrier is subsequently directly transferred onto a carrier material (for example a paper web) at a transfer printing station, or the toner image located on the intermediate carrier is re-supplied to the transfer printing region between photoconductor and intermediate carrier in order to print a further (in particular differently-colored) toner image over the toner image already located on the intermediate carrier. This method of printing toner images one over the other is also designated as pick-up of the toner images in a collection mode. The second toner image can, for example, have a toner color different from that of the first toner image or contain a special toner, in particular a machine-readable microtoner. A two-color print with a base color and an additional color can thereby be generated.
Furthermore, printers and copiers are known in which three or four different-colored toner images are printed over one another in order to thereby obtain a print image in full-color printing. During the pick-up of the toner images, the intermediate carrier is pivoted away from the carrier material such that no contact between the intermediate carrier and the carrier material is present during the collection. Only when all toner images are printed over one another on the intermediate carrier is a mechanical contact produced between the intermediate carrier and the carrier material in order to transfer the complete, collected toner image onto the carrier material. The mechanical contact is advantageously established at the point in time at which the leading edge of the toner image located on the intermediate carrier has reached the transfer printing location for transfer-printing of the toner image from the intermediate carrier onto the carrier material. A cleaning station is subsequently pivoted onto the carrier element when the point at which the leading edge of the transferred toner image on the intermediate carrier has been located and has reached the cleaning station.
Corresponding stress states (i.e. load states) of the intermediate carrier thereby result due to the different operating phases, due to which stress states the circulation speed of the intermediate carrier is changed. The operating phases and the load states resulting from these are subsequently explained in further detail in the Figure descriptions regarding FIGS. 1 through 14b. 
A slippage occurring dependent on the load state results at the drive roller from the different load states. The circulation speed and the circulation time of the intermediate carrier change due to the different slippage at the drive roller. These changes of the circulation speed or circulation time effect a displacement relative to one another of the position of a plurality of successive toner images transferred onto the intermediate carrier as well as the compression of individual toner images or parts of the toner images in the transport direction of the intermediate image carrier.
A print and copier device for performance-adapted monochrome and color one- and two-sided printing of a recording medium is known from the international patent application WO 98/39691 and the U.S. Pat. No. 6,246,856. A plurality of different-colored toner images are thereby generated on a photoconductor belt and subsequently transferred onto a transfer belt on which the toner images are collected before they are transferred all together onto a paper web. The collection and transfer occurs in a start-stop operation of the paper web. In the continuous monochrome printing, the toner images are continuously generated in succession on the photoconductor, and transferred onto the transfer belt whereby the transfer belt in continuous operation directly further transfers a toner image onto the paper web. The contents of the international patent application WO 98/39691 and of the U.S. Pat. No. 6,246,856 are herewith incorporated by reference into the present specification.
Furthermore, in the prior art a plurality of attempts have been made to prevent the position displacement and the length variation of a toner image of the same desired length. It was thus attempted to keep the load change optimally low given the restriction of the transfer belt to a paper web via reduction of the speed difference between paper web and transfer belt. However, depending on the paper properties of the paper web a minimum speed difference is necessary, whereby given a change of the paper type of the paper web to be printed, and in particular of the paper width and the paper thickness, the paper speed, or the speed difference between transfer belt and paper web must be readjusted. An arrangement for reduction load given an activated cleaning unit is known from the German patent document DE 199 42 116 C2. The contents of the patent document DE 199 42 116 C2 as well as the patents or patent applications cited therein is herewith incorporated by reference into the present specification. Due to the arrangement known from this document, the forces acting on the transfer belt which are caused by the cleaning unit are reduced. However, a load change that leads to the disadvantages already described remains upon activation of the cleaning unit.
In the prior art there were also solution approaches to compensate the print image displacement via an adaptation of the write speed by the imaging unit, i.e. by the character generator or the laser exposure device, in that the subsequent position displacement and/or compression or stretching of the toner image is already taken into account in the generation of the latent print image.
Alternatively, solution proposals are known in which the speed-influenced pivot movements occur before or after the toner image generation or after the transfer-printing of the toner image onto the carrier material. However, the overall print speed of the printer is therewith significantly reduced.